A fluorine-containing elastomer is widely used in various fields such as the automobile industry, the semiconductor industry, and the chemical industry as a sealing material used under severe environments since the fluorine-containing elastomer shows excellent chemical resistance, solvent resistance, and heat resistance.
For example in the automobile industry, a fluorine-containing elastomer is used as a sealing material, a hose material and the like in an engine and its peripheral equipments, AT equipments, and a fuel system and its peripheral equipments etc, however, along with environmental regulations in recent year, a regulation of SHED (Sealed Housing for Evaporative Determination) has been reinforced, and development of a fuel rubber material having low fuel permeability is particularly expected. The fuel rubber material is also required to have various properties such as processability, oil resistance and cold resistance other than low fuel permeability.
Further, an oxygen-containing compound is added to recent fuels in order to reduce CO2 emission, however, the oxygen-containing compound acts for promoting fuel permeability of a fluorine containing rubber, thus, development of a material having low fuel permeability is desired for such fuels in which the oxygen-containing compound is added. As for such fuel rubber material, a fluorine-containing rubber excellent in properties such as heat resistance and oil resistance have drawn attention. However, in a fluorine-containing rubber in mainstream polyol cure, when a fluorine amount is increased for low fuel permeability, processability tends to be lowered, and it has been considered that satisfying these properties together is difficult.
As a fluorine-containing elastomer in which processability is improved, a fluorine-containing elastomer comprising a tetrafluoroethylene-propylene copolymer and a vinylidene fluoride-propylene hexafluoride copolymer having a number average molecular weight of at most 5,000 is disclosed in JP-A-61-206115. This fluorine-containing elastomer is improved in processability since a low molecular weight polymer is added, however, there is no particular description regarding a fluorine amount, and since a fluorine amount of any copolymer used in Examples is less than 67%, the fluorine-containing elastomer is not sufficient in respect of low fuel permeability.
JP-A-2-117945 also discloses a composition comprising a fluoroelastomer having a fluorine amount of 68 to 71% by weight and a intrinsic viscosity of 100 to 300 mL/g and a fluoroelastomer having a fluorine amount of 64 to 67% by weight and a intrinsic viscosity of 10 to 50 mL/g. This composition is improved in compression set, mechanical properties and processability, but since a fluorine amount of the low molecular weight fluoroelastomer is lower than a fluorine amount of the high molecular weight fluoroelastomer, the composition is not sufficient in respect of low fuel permeability.
As a general means to improve processability, a process of adding a general plasticizer such as polyethylene and dioctyl sebacate is mentioned. Although it is possible to lower viscosity of an elastomer composition by using a plasticizer, since fuel permeability is deteriorated, the process is not sufficient in respect of low fuel permeability.
As described above, a fluorine-containing elastomer composition improving both processability and fuel permeability has not existed.